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Introduction

In the eighteenth century, as the field of chemistry was evolving from alchemy, a field of the new natural philosophy was created around the idea of air as a reagent. Before this, air was primarily considered a static substance that would not react and simply existed. However, as Lavoisier and several other chymists would insist, the air was indeed dynamic, and would not only be influenced by combusted material, but would also influence the properties of different substances.

The primary concern of pneumatic chemistry was combustion reactions. These reactions would give off different "airs" as chymists would call them, and these different airs contained more simple substances, as combustion reactions are considered an analytical method. Until Lavoisier, these airs were considered separate entities with different properties; Lavoisier was responsible largely for changing the idea of air as being constituted by these different airs that his contemporaries and earlier chymists had discovered.

This study of gases was brought about by Stephen Hale with the invention of the pneumatic trough, an instrument capable of collecting the gas given off by reactions with reproducible results. The term gas was coined by J. B. van Helmont, towards the end of the seventeenth century. This term was derived from the Greek word chaos as a result of his inability to collect properly the substances given off by reactions, as he was the first natural philosopher to make an attempt at carefully studying the third type of matter. However, it was not until Lavoisier performed his research in the eighteenth century that the word was used universally by scientists as a replacement for airs.

The Eighteenth Century

In the eighteenth century, with the rise of combustion analysis in chemistry, Stephen Hales invented the pneumatic trough in order to collect gases from the samples of matter he used; while uninterested in the properties of the gases he collected, he wanted to explore how much gas was given off from the materials he burned or let ferment. Hales was successful in preventing the air from losing it's "elasticity," i.e. preventing it from experiencing a loss in volume, by bubbling the gas through water, and therefore dissolving the soluble gases.

The pneumatic trough was integral thereon in work with gases (or, as contemporary chemists called them, airs). Work done by Joseph Black, Joseph Priestly, Herman Boerhaave, Robert Hooke, and Henry Cavendish revolved largely around the use of the instrument, allowing them to collect airs given off by different chemical reactions and combustion analyses. Their work led to the discovery of a few integral types of airs, such as dephlogisticated air (discovered by Joseph Priestly).

Moreover, the chemistry of airs was not limited to combustion analyses. During the eighteenth century, many chymists used the discovery of airs as a new path for exploring old problems, with one example being the field of medicinal chemistry. One particular Englishman, James Watt, began to take the idea of airs and use them in what was referred to as pneumatic therapy, or the use of airs to make laboratories more workable with fresh airs and also aid patients with different illnesses, with varying degrees of success. Most human experimentation done was performed on the chymists themselves, as they believed that self-experimentation was a necessary part or progressing the field.

Instrumentation

The Pneumatic Trough

Stephen Hales, called the creator of pneumatic chemistry, created the pneumatic trough in 1727. This instrument was widely used by many chemists to explore the properties of different airs, such as what was called inflammable air (what is modernly called hydrogen). Lavoisier used this in addition to his gasometer to collect gases and analyze them, aiding him in creating his list of simple substances.

The pneumatic trough, while integral throughout the eighteenth century, was modified several times to collect gases more efficiently or just to collect more gas. For example, Cavendish noted that the amount of fixed air that was given off by a reaction was not entirely present above the water; this meant that fixed water was absorbing some of this air, and could not be used quantitatively to collect that particular air. So, he replaced the water in the trough with mercury instead, in which most airs were not soluble. By doing so, he could not only collect all airs given off by a reaction, but he could also determine the solubility of airs in water. While this was the major adaptation of the trough in the late eighteenth century, several minor changes were made before and after this substitution of water for mercury, such as adding a shelf to rest the head on while gas collection occurred. This shelf would also allow for less conventional heads to be used, such as Brownrigg's animal bladder.

The Gasometer

During his chemical revolution, Lavoisier created a new instrument for precisely measuring out gases. He called this instrument the gasometer and he had two different versions; the one he used in demonstrations to the Academie and to the public, which was a larger, more expensive version meant to make people believe that it had a large precision, and then another, more lab practical version with a similar precision. This more practical version was cheaper to construct, allowing more chemists to use Lavoisier's instrument.